A commercial annular scintillation camera was purchased and equipped with a multiple pinhole collimator array to create a single photon emission computed tomography (SPECT) scanner for mice. The eight-hole collimator array is rotated mechanically inside the annular scintillation crystal while projecting eight magnified images of the mouse onto the crystal. Performance measurements were carried out with this prototype that verified the design spatial resolution of 1.7 mm and the design senstiivity of 13.8 cps/uCi (CFOV), a combination substantially better than existing small animal SPECT scanners. A signal-processing package was designed to acquire and process data from the Hamamatsu flat panel position-sensitive photomultiplier tube (PSPMT). This tube offers a number of advantages over competing PSPMTs in small animal PET applications but is subject to the degrading effects of pulse pile-up at high rates due to its large field-of-view (48 mm x 48 mm). As a first step in designing analog/digital acquisition electronics that minimize this effect, a simple row and column readout scheme was implemented electronically to obtain baseline test data for future development. Test with this intermediate design show that small cross-section (1.4 mm square) LYSO scintillation crystals can be easily identified at 511 keV with this electronics/PSPMT combination and that the more advanced designs should be able to carry out this same basic task without difficulty. During this reporting period the Imaging Physics Laboratory continued to support the work of multiple intramural research groups in PET imaging studies of tumor hypoxia in mice, gene expression in mice, cerebral stimulation studies in the rat, quantitative methodological validation studies in the rat and mouse and a variety of other biodistribution imaging experiments. Modification and changes to the software of the ATLAS small animal scanner continued to be made in response to requests from these investigators and to enhance the functional capabilities of the system.